1. Field of the Invention
The invention relates to a fairing for the leading edge of fan blading of a turbo jet engine.
2. Summary of the Prior Art
For certain turbo jet engines, the tips of fan blades are subjected to supersonic flow, of the order, for example of Mach 1.5. In order to conform to the regulations, and directions of Certification Authorities with regard to material capable of resisting the bird impact, the thickness of the leading edge of the blades has been exaggerated on the basis of the sole consideration of the strength of the materials and the optimization of the aerodynamic efficiency of the stage.
The excessive thickening of the leading edge gives rise to the creation of a detached shock wave system which results in extended localized subsonic zones and which reduces the efficiency of the compression stage.
The flow of air which enters into the duct "sees" the periodic shock wave system, which results in an amplification effect responsible for appreciable loss in efficiency over the whole of the stage. This effect is more important, in the case of fans with a peripheral heel or platform, employed in certain engines, because of the additional thickening necessitated by the connection of the leading edge to the heel or platform. The platforms together form a peripheral shroud.
Tests and calculations have shown that the generation of losses can be expressed as curves as a function of the Mach number which have an exponential tendency, and it has been estimated that 20% of the total losses of the blading arise for this reason. This part of the losses being is attributable solely to the thickening of the leading edge. According to aerodynamic flow theory these losses result from the detachment of the shock waves at an inclination to the leading edges.
The solution will consist in thinning down the profile of the leading edge, at least at the head of the blade, so as to create inclined shocks without modifying the relative flow which remains supersonic. U.S. Pat. No. 1,862,827 teaches members connected to the leading edges of steam turbine blades which have as their objective the avoidance of erosion resultant from impact produced by water droplets. They have, in the sense of rotation, projections of which the surfaces encounter the droplets at angles which are sufficiently small to avoid the formation of holes under repeated impact action.
Other members, described in U.S. Pat. No. 3,365,126, comprise points on their outer edges, which tightly cover the leading edge of the compressor blades or of the fan of an aircraft engine. Their objective, identical to that of the patent referred to above, is to avoid erosion caused by water drops, ice or dust, as well as to maintain and even to improve the aerodynamic efficiency.